The question I have is why did she cheat on you in the first place, there was certainly a reason for it. Also, giving her the "benefit of the doubt" and having her "prove" to you, how can someone prove that they won't do it again?
Like Brian said, I've had a g/f that has cheated on me too and the biggest thing with me is like Wally said, I couldn't trust her again after that and would always have that "doubt" in the back of my mind. Say if she goes out with friends one night how do you know it's her friends or that other person.
In my opinion it's not worth it man, as I said before there's a reason she did it and until you know that reason you have no clue how to prevent it again, and it might be something that has nothing to do with you. She might have got a wild hair and decided to do it who knows, that's my thoughts on it...